High speed and high efficiency Si-based photodetectors using waveguides formed with silicide for near IR applications

ABSTRACT

According to this invention, silicon-based photodetectors using waveguides formed with silicide regions can have high speed and high efficiency for near IR applications. Utilizing the unique properties of silicides, the proposed method provides a simple and elegant way to implement a photodetector design in which photogenerated carriers travel perpendicular to the direction of light propagation. Therefore, the speed and quantum efficiency of the photodetector may be optimized independently. This device configuration may be implemented in one of the two approaches: (a) waveguides formed through surface silicidation of a silicon-based layer of a substrate (b) waveguides formed through silicidation of ridge waveguide side-walls of a silicon-based layer of a substrate; The use of mature silicon technology promises low cost of production and other benefits.

This application claims the benefit of U.S. Provisional Application No.60/257,285, filed Dec. 26, 2000.

FIELD OF THE INVENTION

This invention pertains to the field of semiconductor photodetectors,and in particular, to Si-based photodetectors enhanced by implementationof silicide waveguides for high speed and high quantum efficiency lightdetection.

BACKGROUND OF THE INVENTION

High-speed detectors and detector arrays are required fortelecommunication systems, for high-capacity local area networks, andfor instrumentation. For chip-to-chip or processor-to-processor dataexchange, optical interconnects will eventually replace today'selectrical methods. Many such approaches are being explored. As thenumber of detectors required for each application increases, lowmaterial cost, mature fabrication technology and compact design presentmany advantages. Compared to detectors requiring surface normalcoupling, waveguide detectors are simpler to assemble.

Traditionally photodetectors with high speed and high quantum efficiencyhave been limited to materials with direct band-gap, such as III–Vcompounds. It is desirable to fabricate such devices with silicon-basedmaterials, to benefit from the low cost and mature silicon technology.Because the light absorption in the near IR range for silicon (Si) islow it is necessary to uncouple the optical path length (quantumefficiency factor) and the distance between electrodes (speed limitfactor).

Metal-semiconductor-metal (MSM) detectors are often proposed as analternative for high-speed operation, as the speed is primarily limitedby the transit time between electrodes. The distance betweenphotodetector electrodes can be scaled down through fine linelithography. MSM detectors are described in the following publications:“140 GHz metal-semiconductor-metal photodetectors onsilicon-on-insulator substrate with a scaled active layer”, M. Y. Liu,E. Chen, and S. Y. Chou, Appl. Phys. Lett. 65(7), p. 887, 1994;“High-speed metal-semiconductor-metal photodetectors fabricated onSOI-substrates”, K. Honkanen, N. Hakkarainen, K. Määttä, A. Kilpelä andP. Kuivalainen, Physica scripta T79, p. 127, 1999; “Comparison of thepicosecond characteristics of silicon and silicon-on-sapphiremetal-semiconductor-metal photodiodes”, C. C. Wang, S. Alexandrou, D.Jacobs-Perkins, and T. Y. Hsinag, Appl. Phys. Lett. 64(26), p.3578,1994. The contents of the aforementioned publications are incorporatedhere in by reference.

To increase the optical path length, and therefore the quantumefficiency, several approaches have been explored. Waveguide geometries,often formed on silicon-on-insulator (SOI) substrates as silicon ridges,have been used, as described in “SOI waveguide GeSi avalanche pinphotodetector at 1.3 μm wavelength”, T. Yoshimoto, S. G. Thomas, K. L.Wang and B. Jalali, IEICE Trans. Electron., E81-C(10), p. 1667, 1998,and “Near-infrared waveguide photodetectors based on polycrystalline Geon silicon-on-insulator substrates” G. Masini, L Colace, G. Assanto,Optical Materials 17 (2001) 243–246, incorporated here in by reference.The collection of carriers is accomplished either by forming MSMstructures on the surface of the ridge waveguides, or by grown-in pinstructures.

Another approach is to use vertical cavity structures, which consist ofa thin absorbing layer sandwiched between two mirrors. In the past,vertical cavities have been used successfully to build high speed, highquantum efficiency photodetectors. The mirrors were made ofdielectric/silicon or Si/SiGe multi-layers through deposition. Thismethod is described in the following publications: “Selective epitaxialgrowth Si resonant cavity photodetector”, G. W. Neudeck, J. Denton, J.Qi, J. D. Schaub, R. Li and J. C. Campbell, IEEE Photo. Technol. Lett.10(1), p. 129 (1998) and “Si/SiO₂ resonant cavity photodetector”, D. C.Diaz, C. L. Schow, Jieming Qi, J. C. Campbell, J. C. Bean and L. J.Peticolas, Appl. Phys. Lett. 69(19), p. 2798 (1996).

Buried silicide layers, embedded by wafer bonding or implant, have alsobeen used as mirror material for vertical cavities, as described in: “Avertical cavity longwave infrared SiGe/Si photodetector using a buriedsilicide mirror,” R. T. Carline, D. A. O. Hope, V. Nayar, D. J. Robinsand M. B. Stanaway, Technique Digest of IEDM'97, p. 36.1.1 (1997) and in“Fabrication of ultra-fast Si-based MSM photodetector,” M. Löken, L., S.Mantel and Ch. Buchal, Electron. Lett. 34 (10), p. 1027, 1998. Thecontents of the aforementioned publications are incorporated here in byreference.

However, the devices described in the prior art present severallimitations described below.

In the case of MSM detectors, only the photocarriers generated near theelectrodes (i.e. near the surface) can be collected through drift in theelectric field. For carriers generated outside the electric field, aportion of the carriers can be collected through diffusion rather thatdrift, resulting in a reduction in speed. Forming MSM structures on SOIsubstrates with thin silicon layers may eliminate this problem. However,the responsivity becomes very low.

Waveguide structures with vertical pin junctions require a specificlayer sequence, therefore are costly to make and can limit thepossibilities for component integration.

For the vertical cavity structures, light is coupled in from the surfacenormal direction, and these detectors operate only at a set of discreteresonant wavelengths. The resonant wavelength is determined by thedistance between the two mirror surfaces. To obtain high quantumefficiency at the designed wavelength, stringent control in layerthickness is required, which often is a challenge in itself.

In view of the potential advantages of a Si-based photodetector, itwould be very advantageous to provide a relatively simple Si-basedphotodetector with high quantum efficiency and fast response. Thisapplication discloses such is a photodetector.

SUMMARY OF THE INVENTION

It has been long desired to design photodetectors in whichphotogenerated carriers travel perpendicular to the direction of thelight propagation, so that speed and quantum efficiency may be optimizedindependently. Utilizing the unique properties of silcides, the proposedmethod provides a simple and elegant way to implement such designs.

The first aspect of the invention is a photodetector comprising twoseparated silicide regions on a substrate and a waveguide of asilicon-based material formed between side-walls of the two silicideregions.

The second aspect of the invention is a photodetector comprising twoseparated silicide regions on a substrate and a wavegnide of asilicon-based material formed between side-walls of the two silicideregions, wherein the silicon-based material is one of a group ofmaterials comprising: silicon, amorphous silicon, silicon germanium, andamorphous silicon germanium.

The third aspect of the invention is a photodetector comprising twoseparated silicide regions on a substrate and a waveguide of asilicon-based material formed between side-walls of the two silicideregions, wherein the two silicide regions are produced using a metalfrom a group of metals comprising: nickel, platinum, tungsten, andcobalt.

The fourth aspect of the invention relates to a surface silicidationmethod of producing a photodetector having a waveguide of asilicon-based material, comprising steps of:

-   a/ depositing a metal layer on a silicon-based material layer of a    substrate;-   b/ etching to selectively remove unwanted regions of the metal    layer; and-   c/ heating the metal layer to induce a metal-silicon reaction to    produce at least two separated silicide regions, the silicide    regions forming the waveguide of silicon-based material    therebetween.

The fifth aspect of the invention relates to a ridge side-wallssilicidation method of producing a photodetector having a waveguide of asilicon-based material, comprising steps of:

-   a/ forming a ridge in the silicon-based material layer of a    substrate and applying a mask on top of the ridge;-   b/ depositing a metal layer on the silicon-based material layer of    the substrate;-   c/ heating the metal layer to induce a metal-silicon reaction to    produce at least two separated silicide regions, the silicide    regions forming the waveguide therebetween; and-   d/ etching to selectively remove unwanted metal from the mask    without affecting the silicide regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a–c relate to the surface silicidation process.

FIG. 1 a is a cross-section view of a SOI wafer before a silicidationprocess.

FIG. 1 b is a cross-section view of a SOI wafer after the silicidationprocess.

FIG. 1 c is a top view of the silicon-based photodetector with silicidewaveguides.

FIGS. 2 a–b, 3 a–b, and 4 a–b relate to the silicon-based material ridgeside-walls silicidation process.

FIG. 2 a is a cross-section view of a SOI wafer with a silicon ridgewaveguide before the silicidation process.

FIG. 2 b is a cross-section view of a SOI wafer as in FIG. 2 a after thesilicidation process.

FIG. 3 a is a cross-section view of a Si substrate or CMOS circuits withan amorphous Si-based material ridge waveguide before the silicidationprocess.

FIG. 3 b is a cross-section view of a Si substrate or CMOS circuits asin

FIG. 3 a after the silicidation process.

FIG. 4 a is a cross-section view of a Si substrate with a Si/SiGe basedmaterial ridge waveguide before the silicidation process.

FIG. 4 b is a cross-section view of a Si substrate as in FIG. 4 a afterthe silicidation process.

FIG. 5 is a cross-section view of a photodetector with a Si/SiGe basedmaterial layer structure.

FIG. 6 is a cross-section view of a photodetector with an amorphous SiGebased layer material structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention of this application may be implemented by using thefollowing two approaches:

-   (A) Waveguides Formed Through Surface Silicidation on a SOI    Substrate (FIG. 1 a–c)

A metal layer 1 is first deposited on a SOI wafer 2. The unwantedregions of the metal are removed through etching techniques. The waferis then heated in an atmosphere such as nitrogen. During the first stageof the heating process lasting approximately 90 seconds the initialtemperature is approximately 400° C. and is gradually raised to 475° C.During the next stage the wafer is heated at approximately 550° C. fortwo minutes. Under such conditions the chosen metal, currently platinum,nickel, tungsten, or cobalt, reacts with the top silicon layer 3 of theSOI wafer 2 and forms silicide regions 4. The metal thickness can betuned to consume the entire depth (usually up to 1 μm) of the siliconlayer 3. The silicon region 5 bound by the inner side-walls of thesilicide regions 4 becomes a waveguide in the lateral direction, withthe side-walls acting as mirrors between which light is confined. Lightcoming from a fiber-optic cable 6 is coupled in from side facets. Atapered input waveguide 7 may also be incorporated to improve thecoupling efficiency.

Since silicides are metals with high conductivity, they can also serveas electrodes (FIG. 1 c). For photodetector applications, the siliconlayer should be lightly n-type doped. The silicide should possesssufficiently large Schottky barrier height to form a Schottky contactwith silicon. Several types of silicides, such as platinum silicide(PtSi) and nickel silicide (NiSi), as well as silicides of tungsten (W)and cobalt (Co), could serve this purpose. The photo carriers generatedare collected by the silicide regions acting as electrodes. When thedistance between the mirrors is small enough (about 1 μm), the siliconlayer between the electrodes is fully depleted under moderate bias. Thetransit time is therefore limited only by the distance between theelectrodes and the saturation velocity in silicon. As the entire volumeof active silicon can be biased into depletion, this photodetector doesnot suffer from the diffusion time limitation of conventional MSMdetectors.

-   (B) Waveguides Formed Through Silicidation of Ridge Waveguide    Side-Walls on a SOI Substrate (FIG. 2 a–b)

A silicon ridge waveguide 8 is formed on a SOI substrate 2. Silicondioxide (SiO₂) layer 9 and photoresist layer 10 are applied on its topsurface as an etching mask (FIG. 2 a). Next, a layer of metal 11 isdeposited on the surface of the wafer and the sample is subjected to asilicidation process similar to that described in approach (A). Sincethe metal 11 on top of the ridge is separated from silicon by the SiO₂layer 9 it does not become silicide. It can be removed by selectiveetching solutions, which etch the metal but do not etch the silicides.The L-shaped silicide regions 13 at both sides of the waveguide 12 arenow electrically isolated (FIG. 2 b). Small separation between theelectrodes can be easily achieved. As discussed above, the silicideregions serve both as mirrors and electrodes. This method is moresuitable for structures with thicker silicon layers.

Approaches (A) and (B) may also be implemented with amorphoussilicon-based material (FIG. 3 a–b). The amorphous silicon-basedmaterial 14 is deposited on a Si-based substrate or on CMOS circuits 15covered with a dielectric film 16 (silicon dioxide or silicone nitride).The dielectric layer is needed so that the amorphous layer does notdisturb the Si crystaline or other structure of the substrate.Subsequently both (A) and (B) methods can be used to produce silicidewaveguide regions 17. By using amorphous silicon-based material, theease of integration is greatly improved.

It has been determined that certain metals can react with silicongermanium

(SiGe) and form silicides under appropriate conditions. The use of SiGeto produce silicides makes photodetectors suitable for detecting longerwavelengths. If SiGe photodetectors can be demonstrated to operate inthe 1.3–1.55 μm range, they will have major application intelecommunication networks. Both approaches described in (A) and (B)could be applied to SiGe materials. FIGS. 4 a–b show the structures ofthe photodetector with Si/SiGe based material layer 18 before and afterthe silicidation process. SiGe material may be used as superlattices oras alloy. In FIG. 5 the Si/SiGe layer 19 shows a structure ofepitaxially grown SiGe superlattices 20. Although the thickness of suchstructure is limited, it can provide longer wavelength absorption edge,and lower leakage current.

Amorphous silicon germanium material could also be used to producesilicides. Amorphous SiGe may exist as uniform SiGe alloy layer, whichis easy to grow but gives higher leakage current. FIG. 6 shows across-section view of a photodetector with a layer of an amorphous SiGealloy 21 with another layer of silicon 22.

The main metals used for silicidation processes for SiGe based materialsare nickel and platinum. For both metals, the temperature required forsilicidation is low enough (around 500° C.) not to deteriorate thestrained SiGe layer quality. The application of other metals to formsilicides has to be further researched.

Only standard VLSI technologies are required to fabricate the proposedphotodetector devices, with very few process steps involved. Thetechnology is also fully compatible with VLSI manufacturing. Proposedwaveguide photodetector design is also more suitable for integrationinto fiber optic configurations.

1. A photodetector comprising: two separated suicide regions on asubstrate; and a waveguide of a silicon-based material having sidewalls,each sidewall being entirely and continuously in contact with one of thesilicide regions, such that said sidewalls provide lateral confinementof the waveguide, wherein said two separated silicide regions serve asself-contained electrodes forming a Schottky contact with each sidewallof the waveguide, thereby imposing substantially a uniform electricalfield that is localized in the waveguide.
 2. The photodetector accordingto claim 1, wherein said silicon-based material is one of a group ofmaterials comprising: silicon, amorphous silicon, silicon germanium, andamorphous silicon germanium.
 3. The photodetector according to claim 1,wherein said two separated silicide regions are produced using a metalfrom a group of metals comprising: nickel, platinum, tungsten, andcobalt.
 4. The photodetector according to claim 1, wherein saidphotodetector has a tapered input waveguide.
 5. The photodetectoraccording to claim 1, wherein said waveguide is located on saidsubstrate.